Routing Working Group F. Baker
Internet-Draft
Intended status: Informational C. Bowers
Expires: November 16, 2018 Juniper Networks
J. Linkova
Google
May 15, 2018
Enterprise Multihoming using Provider-Assigned Addresses without Network
Prefix Translation: Requirements and Solution
draft-ietf-rtgwg-enterprise-pa-multihoming-06
Abstract
Connecting an enterprise site to multiple ISPs using provider-
assigned addresses is difficult without the use of some form of
Network Address Translation (NAT). Much has been written on this
topic over the last 10 to 15 years, but it still remains a problem
without a clearly defined or widely implemented solution. Any
multihoming solution without NAT requires hosts at the site to have
addresses from each ISP and to select the egress ISP by selecting a
source address for outgoing packets. It also requires routers at the
site to take into account those source addresses when forwarding
packets out towards the ISPs.
This document attempts to define a complete solution to this problem.
It covers the behavior of routers to forward traffic taking into
account source address, and it covers the behavior of host to select
appropriate source addresses. It also covers any possible role that
routers might play in providing information to hosts to help them
select appropriate source addresses. In the process of exploring
potential solutions, this documents also makes explicit requirements
for how the solution would be expected to behave from the perspective
of an enterprise site network administrator .
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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expensive for the enterprise relative to using provider-independent
addresses. PA multihoming is also a practice that should be
facilitated and encouraged because it does not add to the size of the
Internet routing table, whereas PI multihoming does. Note that PA is
also used to mean "provider-aggregatable". In this document we
assume that provider-assigned addresses are always provider-
aggregatable.
With PA multihoming, for each ISP connection, the site is assigned a
prefix from within an address block allocated to that ISP by its
National or Regional Internet Registry. In the simple case of two
ISPs (ISP-A and ISP-B), the site will have two different prefixes
assigned to it (prefix-A and prefix-B). This arrangement is
problematic. First, packets with the "wrong" source address may be
dropped by one of the ISPs. In order to limit denial of service
attacks using spoofed source addresses, BCP38 [RFC2827] recommends
that ISPs filter traffic from customer sites to only allow traffic
with a source address that has been assigned by that ISP. So a
packet sent from a multihomed site on the uplink to ISP-B with a
source address in prefix-A may be dropped by ISP-B.
However, even if ISP-B does not implement BCP38 or ISP-B adds
prefix-A to its list of allowed source addresses on the uplink from
the multihomed site, two-way communication may still fail. If the
packet with source address in prefix-A was sent to ISP-B because the
uplink to ISP-A failed, then if ISP-B does not drop the packet and
the packet reaches its destination somewhere on the Internet, the
return packet will be sent back with a destination address in prefix-
A. The return packet will be routed over the Internet to ISP-A, but
it will not be delivered to the multihomed site because its link with
ISP-A has failed. Two-way communication would require some
arrangement for ISP-B to advertise prefix-A when the uplink to ISP-A
fails.
Note that the same may be true with a provider that does not
implement BCP 38, if his upstream provider does, or has no
corresponding route. The issue is not that the immediate provider
implements ingress filtering; it is that someone upstream does, or
lacks a route.
With IPv4, this problem is commonly solved by using [RFC1918] private
address space within the multi-homed site and Network Address
Translation (NAT) or Network Address/Port Translation (NAPT) on the
uplinks to the ISPs. However, one of the goals of IPv6 is to
eliminate the need for and the use of NAT or NAPT. Therefore,
requiring the use of NAT or NAPT for an enterprise site to multihome
with provider-assigned addresses is not an attractive solution.
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[RFC6296] describes a translation solution specifically tailored to
meet the requirements of multi-homing with provider-assigned IPv6
addresses. With the IPv6-to-IPv6 Network Prefix Translation (NPTv6)
solution, within the site an enterprise can use Unique Local
Addresses [RFC4193] or the prefix assigned by one of the ISPs. As
traffic leaves the site on an uplink to an ISP, the source address
gets translated to an address within the prefix assigned by the ISP
on that uplink in a predictable and reversible manner. [RFC6296] is
currently classified as Experimental, and it has been implemented by
several vendors. See Section 6.2, for more discussion of NPTv6.
This document defines routing requirements for enterprise multihoming
using provider-assigned IPv6 addresses. We have made no attempt to
write these requirements in a manner that is agnostic to potential
solutions. Instead, this document focuses on the following general
class of solutions.
Each host at the enterprise has multiple addresses, at least one from
each ISP-assigned prefix. Each host, as discussed in Section 4.1 and
[RFC6724], is responsible for choosing the source address applied to
each packet it sends. A host SHOULD be able respond dynamically to
the failure of an uplink to a given ISP by no longer sending packets
with the source address corresponding to that ISP. Potential
mechanisms for the communication of changes in the network to the
host are Neighbor Discovery Router Advertisements, DHCPv6, and
ICMPv6.
The routers in the enterprise network are responsible for ensuring
that packets are delivered to the "correct" ISP uplink based on
source address. This requires that at least some routers in the site
network are able to take into account the source address of a packet
when deciding how to route it. That is, some routers must be capable
of some form of Source Address Dependent Routing (SADR), if only as
described in [RFC3704]. At a minimum, the routers connected to the
ISP uplinks (the site exit routers or SERs) must be capable of Source
Address Dependent Routing. Expanding the connected domain of routers
capable of SADR from the site exit routers deeper into the site
network will generally result in more efficient routing of traffic
with external destinations.
The document first looks in more detail at the enterprise networking
environments in which this solution is expected to operate. It then
discusses existing and proposed mechanisms for hosts to select the
source address applied to packets. Finally, it looks at the
requirements for routing that are needed to support these enterprise
network scenarios and the mechanisms by which hosts are expected to
select source addresses dynamically based on network state.
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Internet-Draft Enterprise PA Multihoming May 20182. Enterprise Multihoming Requirements2.1. Simple ISP Connectivity with Connected SERs
We start by looking at a scenario in which a site has connections to
two ISPs, as shown in Figure 1. The site is assigned the prefix
2001:db8:0:a000::/52 by ISP-A and prefix 2001:db8:0:b000::/52 by ISP-
B. We consider three hosts in the site. H31 and H32 are on a LAN
that has been assigned subnets 2001:db8:0:a010::/64 and
2001:db8:0:b010::/64. H31 has been assigned the addresses
2001:db8:0:a010::31 and 2001:db8:0:b010::31. H32 has been assigned
2001:db8:0:a010::32 and 2001:db8:0:b010::32. H41 is on a different
subnet that has been assigned 2001:db8:0:a020::/64 and
2001:db8:0:b020::/64.
2001:db8:0:1234::101 H101
|
|
2001:db8:0:a010::31 --------
2001:db8:0:b010::31 ,-----. / \
+--+ +--+ +----+ ,' `. : :
+---|R1|---|R4|---+---|SERa|-+ ISP-A +--+-- :
H31--+ +--+ +--+ | +----+ `. ,' : :
| | `-----' : Internet :
| | : :
| | : :
| | : :
| | ,-----. : :
H32--+ +--+ | +----+ ,' `. : :
+---|R2|----------+---|SERb|-+ ISP-B +--+-- :
+--+ | +----+ `. ,' : :
| `-----' : :
| : :
+--+ +--+ +--+ \ /
H41------|R3|--|R5|--|R6| --------
+--+ +--+ +--+
2001:db8:0:a020::41
2001:db8:0:b020::41
Figure 1: Simple ISP Connectivity With Connected SERs
We refer to a router that connects the site to an ISP as a site edge
router(SER). Several other routers provide connectivity among the
internal hosts (H31, H32, and H41), as well as connecting the
internal hosts to the Internet through SERa and SERb. In this
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example SERa and SERb share a direct connection to each other. In
Section 2.2, we consider a scenario where this is not the case.
For the moment, we assume that the hosts are able to make good
choices about which source addresses through some mechanism that
doesn't involve the routers in the site network. Here, we focus on
primary task of the routed site network, which is to get packets
efficiently to their destinations, while sending a packet to the ISP
that assigned the prefix that matches the source address of the
packet. In Section 4, we examine what role the routed network may
play in helping hosts make good choices about source addresses for
packets.
With this solution, routers will need form of Source Address
Dependent Routing, which will be new functionality. It would be
useful if an enterprise site does not need to upgrade all routers to
support the new SADR functionality in order to support PA multi-
homing. We consider if this is possible and what are the tradeoffs
of not having all routers in the site support SADR functionality.
In the topology in Figure 1, it is possible to support PA multihoming
with only SERa and SERb being capable of SADR. The other routers can
continue to forward based only on destination address, and exchange
routes that only consider destination address. In this scenario,
SERa and SERb communicate source-scoped routing information across
their shared connection. When SERa receives a packet with a source
address matching prefix 2001:db8:0:b000::/52 , it forwards the packet
to SERb, which forwards it on the uplink to ISP-B. The analogous
behaviour holds for traffic that SERb receives with a source address
matching prefix 2001:db8:0:a000::/52.
In Figure 1, when only SERa and SERb are capable of source address
dependent routing, PA multi-homing will work. However, the paths
over which the packets are sent will generally not be the shortest
paths. The forwarding paths will generally be more efficient as more
routers are capable of SADR. For example, if R4, R2, and R6 are
upgraded to support SADR, then can exchange source-scoped routes with
SERa and SERb. They will then know to send traffic with a source
address matching prefix 2001:db8:0:b000::/52 directly to SERb,
without sending it to SERa first.
2.2. Simple ISP Connectivity Where SERs Are Not Directly Connected
In Figure 2, we modify the topology slightly by inserting R7, so that
SERa and SERb are no longer directly connected. With this topology,
it is not enough to just enable SADR routing on SERa and SERb to
support PA multi-homing. There are two solutions to ways to enable
PA multihoming in this topology.
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general guiding principle, we assume an enterprise network operator
will expect a multihomed network to behave as close as to a single-
homed network as possible. So a solution that meets those
expectations where possible is a good thing.
For traffic between internal hosts and traffic from outside the site
to internal hosts, an enterprise network operator would expect there
to be no visible change in the path taken by this traffic, since this
traffic does not need to be routed in a way that depends on source
address. It is also reasonable to expect that internal hosts should
be able to communicate with each other using either of their source
addresses without restriction. For example, H31 should be able to
communicate with H41 using a packet with S=2001:db8:0:a010::31,
D=2001:db8:0:b010::41, regardless of the state of uplink to ISP-B.
These goals can be accomplished by having all of the routers in the
network continue to originate normal unscoped destination routes for
their connected networks. If we can arrange so that these unscoped
destination routes get used for forwarding this traffic, then we will
have accomplished the goal of keeping forwarding of traffic destined
for internal hosts, unaffected by the multihoming solution.
For traffic destined for external hosts, it is reasonable to expect
that traffic with an source address from the prefix assigned by ISP-A
to follow the path to that the traffic would follow if there is no
connection to ISP-B. This can be accomplished by having SERa
originate a source-scoped route of the form (S=2001:db8:0:a000::/52,
D=::/0) . If all of the routers in the site support SADR, then the
path of traffic exiting via ISP-A can match that expectation. If
some routers don't support SADR, then it is reasonable to expect that
the path for traffic exiting via ISP-A may be different within the
site. This is a tradeoff that the enterprise network operator may
decide to make.
It is important to understand how this multihoming solution behaves
when an uplink to one of the ISPs fails. To simplify this
discussion, we assume that all routers in the site support SADR. We
first start by looking at how the network operates when the uplinks
to both ISP-A and ISP-B are functioning properly. SERa originates a
source-scoped route of the form (S=2001:db8:0:a000::/52, D=::/0), and
SERb is originates a source-scoped route of the form
(S=2001:db8:0:b000::/52, D=::/0). These routes are distributed
through the routers in the site, and they establish within the
routers two set of forwarding paths for traffic leaving the site.
One set of forwarding paths is for packets with source address in
2001:db8:0:a000::/52. The other set of forwarding paths is for
packets with source address in 2001:db8:0:b000::/52. The normal
destination routes which are not scoped to these two source prefixes
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play no role in the forwarding. Whether a packet exits the site via
SERa or via SERb is completely determined by the source address
applied to the packet by the host. So for example, when host H31
sends a packet to host H101 with (S=2001:db8:0:a010::31,
D=2001:db8:0:1234::101), the packet will only be sent out the link
from SERa to ISP-A.
Now consider what happens when the uplink from SERa to ISP-A fails.
The only way for the packets from H31 to reach H101 is for H31 to
start using the source address for ISP-B. H31 needs to send the
following packet: (S=2001:db8:0:b010::31, D=2001:db8:0:1234::101).
This behavior is very different from the behavior that occurs with
site multihoming using PI addresses or with PA addresses using NAT.
In these other multi-homing solutions, hosts do not need to react to
network failures several hops away in order to regain Internet
access. Instead, a host can be largely unaware of the failure of an
uplink to an ISP. When multihoming with PA addresses and NAT,
existing sessions generally need to be re-established after a failure
since the external host will receive packets from the internal host
with a new source address. However, new sessions can be established
without any action on the part of the hosts.
Another example where the behavior of this multihoming solution
differs significantly from that of multihoming with PI address or
with PA addresses using NAT is in the ability of the enterprise
network operator to route traffic over different ISPs based on
destination address. We still consider the fairly simple network of
Figure 2 and assume that uplinks to both ISPs are functioning.
Assume that the site is multihomed using PA addresses and NAT, and
that SERa and SERb each originate a normal destination route for
D=::/0, with the route origination dependent on the state of the
uplink to the respective ISP.
Now suppose it is observed that an important application running
between internal hosts and external host H101 experience much better
performance when the traffic passes through ISP-A (perhaps because
ISP-A provides lower latency to H101.) When multihoming this site
with PI addresses or with PA addresses and NAT, the enterprise
network operator can configure SERa to originate into the site
network a normal destination route for D=2001:db8:0:1234::/64 (the
destination prefix to reach H101) that depends on the state of the
uplink to ISP-A. When the link to ISP-A is functioning, the
destination route D=2001:db8:0:1234::/64 will be originated by SERa,
so traffic from all hosts will use ISP-A to reach H101 based on the
longest destination prefix match in the route lookup.
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Implementing the same routing policy is more difficult with the PA
multihoming solution described in this document since it doesn't use
NAT. By design, the only way to control where a packet exits this
network is by setting the source address of the packet. Since the
network cannot modify the source address without NAT, the host must
set it. To implement this routing policy, each host needs to use the
source address from the prefix assigned by ISP-A to send traffic
destined for H101. Mechanisms have been proposed to allow hosts to
choose the source address for packets in a fine grained manner. We
will discuss these proposals in Section 4. However, interacting with
host operating systems in some manner to ensure a particular source
address is chosen for a particular destination prefix is not what an
enterprise network administrator would expect to have to do to
implement this routing policy.
2.4. More complex ISP connectivity
The previous sections considered two variations of a simple
multihoming scenario where the site is connected to two ISPs offering
only Internet connectivity. It is likely that many actual enterprise
multihoming scenarios will be similar to this simple example.
However, there are more complex multihoming scenarios that we would
like this solution to address as well.
It is fairly common for an ISP to offer a service in addition to
Internet access over the same uplink. Two variation of this are
reflected in Figure 3. In addition to Internet access, ISP-A offers
a service which requires the site to access host H51 at
2001:db8:0:5555::51. The site has a single physical and logical
connection with ISP-A, and ISP-A only allows access to H51 over that
connection. So when H32 needs to access the service at H51 it needs
to send packets with (S=2001:db8:0:a010::32, D=2001:db8:0:5555::51)
and those packets need to be forward out the link from SERa to ISP-A.
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As discussed before, we rely completely on the internal host to set
the source address of the packet properly. In the case of a packet
sent by H31 to access the service in ISP-B at H61, we expect the
packet to have the following addresses: (S=2001:db8:0:b010::31,
D=2001:db8:0:6666::61). The routed network has two potential ways of
distributing routes so that this packet exits the site on the uplink
at SERb2.
We could just rely on normal destination routes, without using
source-prefix scoped routes. If we have SERb2 originate a normal
unscoped destination route for D=2001:db8:0:6666::/64, the packets
from H31 to H61 will exit the site at SERb2 as desired. We should
not have to worry about SERa needing to originate the same route,
because ISP-B should choose a globally unique prefix for the service
at H61.
The alternative is to have SERb2 originate a source-prefix-scoped
destination route of the form (S=2001:db8:0:b000::/52,
D=2001:db8:0:6666::/64). From a forwarding point of view, the use of
the source-prefix-scoped destination route would result in traffic
with source addresses corresponding only to ISP-B being sent to
SERb2. Instead, the use of the unscoped destination route would
result in traffic with source addresses corresponding to ISP-A and
ISP-B being sent to SERb2, as long as the destination address matches
the destination prefix. It seems like either forwarding behavior
would be acceptable.
However, from the point of view of the enterprise network
administrator trying to configure, maintain, and trouble-shoot this
multihoming solution, it seems much clearer to have SERb2 originate
the source-prefix-scoped destination route correspond to the service
offered by ISP-B. In this way, all of the traffic leaving the site
is determined by the source-prefix-scoped routes, and all of the
traffic within the site or arriving from external hosts is determined
by the unscoped destination routes. Therefore, for this multihoming
solution we choose to originate source-prefix-scoped routes for all
traffic leaving the site.
2.5. ISPs and Provider-Assigned Prefixes
While we expect that most site multihoming involves connecting to
only two ISPs, this solution allows for connections to an arbitrary
number of ISPs to be supported. However, when evaluating scalable
implementations of the solution, it would be reasonable to assume
that the maximum number of ISPs that a site would connect to is five.
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It is also useful to note that the prefixes assigned to the site by
different ISPs will not overlap. This must be the case , since the
provider-assigned addresses have to be globally unique.
2.6. Simplified Topologies
The topologies of many enterprise sites using this multihoming
solution may in practice be simpler than the examples that we have
used. The topology in Figure 1 could be further simplified by having
all hosts directly connected to the LAN connecting the two site exit
routers, SERa and SERb. The topology could also be simplified by
having the uplinks to ISP-A and ISP-B both connected to the same site
exit router. However, it is the aim of this draft to provide a
solution that applies to a broad a range of enterprise site network
topologies, so this draft focuses on providing a solution to the more
general case. The simplified cases will also be supported by this
solution, and there may even be optimizations that can be made for
simplified cases. This solution however needs to support more
complex topologies.
We are starting with the basic assumption that enterprise site
networks can be quite complex from a routing perspective. However,
even a complex site network can be multihomed to different ISPs with
PA addresses using IPv4 and NAT. It is not reasonable to expect an
enterprise network operator to change the routing topology of the
site in order to deploy IPv6.
3. Generating Source-Prefix-Scoped Forwarding Tables
So far we have described in general terms how the routers in this
solution that are capable of Source Address Dependent Routing will
forward traffic using both normal unscoped destination routes and
source-prefix-scoped destination routes. Here we give a precise
method for generating a source-prefix-scoped forwarding table on a
router that supports SADR.
1. Compute the next-hops for the source-prefix-scoped destination
prefixes using only routers in the connected SADR domain. These
are the initial source-prefix-scoped forwarding table entries.
2. Compute the next-hops for the unscoped destination prefixes using
all routers in the IGP. This is the unscoped forwarding table.
3. Augment each less specific source-prefix-scoped forwarding table
with all more specific source-prefix-scoped forwarding tables
entries based on the following rule. If the destination prefix
of the less specific source-prefix-scoped forwarding entry
exactly matches the destination prefix of an existing more
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specific source-prefix-scoped forwarding entry (including
destination prefix length), then do not add the less specific
source-prefix-scoped forwarding entry. If the destination prefix
does NOT match an existing entry, then add the entry to the more
source-prefix-scoped forwarding table. As the unscoped
forwarding table is considered to be scoped to ::/0 this process
starts with propagating routes from the unscoped forwarding table
to source-prefix-scoped forwarding tables and then continues with
propagating routes to more-specific-source-prefix-scoped
forwarding tables should they exist.
The forward tables produced by this process are used in the following
way to forward packets.
1. Select the most specific (longerst prefix match) source-prefix-
scoped forwarding table that matches the source address of the
packet (again, the unscoped forwarding table is considered to be
scoped to ::/0).
2. Look up the destination address of the packet in the selected
forwarding table to determine the next-hop for the packet.
The following example illustrates how this process is used to create
a forwarding table for each provider-assigned source prefix. We
consider the multihomed site network in Figure 3. Initially we
assume that all of the routers in the site network support SADR.
Figure 4 shows the routes that are originated by the routers in the
site network.
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Routes originated by SERa:
(S=2001:db8:0:a000::/52, D=2001:db8:0:5555/64)
(S=2001:db8:0:a000::/52, D=::/0)
(D=2001:db8:0:5555::/64)
(D=::/0)
Routes originated by SERb1:
(S=2001:db8:0:b000::/52, D=::/0)
(D=::/0)
Routes originated by SERb2:
(S=2001:db8:0:b000::/52, D=2001:db8:0:6666::/64)
(D=2001:db8:0:6666::/64)
Routes originated by R1:
(D=2001:db8:0:a010::/64)
(D=2001:db8:0:b010::/64)
Routes originated by R2:
(D=2001:db8:0:a010::/64)
(D=2001:db8:0:b010::/64)
Routes originated by R3:
(D=2001:db8:0:a020::/64)
(D=2001:db8:0:b020::/64)
Figure 4: Routes Originated by Routers in the Site Network
Each SER originates destination routes which are scoped to the source
prefix assigned by the ISP that the SER connects to. Note that the
SERs also originate the corresponding unscoped destination route.
This is not needed when all of the routers in the site support SADR.
However, it is required when some routers do not support SADR. This
will be discussed in more detail later.
We focus on how R8 constructs its source-prefix-scoped forwarding
tables from these route advertisements. R8 computes the next hops
for destination routes which are scoped to the source prefix
2001:db8:0:a000::/52. The results are shown in the first table in
Figure 5. (In this example, the next hops are computed assuming that
all links have the same metric.) Then, R8 computes the next hops for
destination routes which are scoped to the source prefix
2001:db8:0:b000::/52. The results are shown in the second table in
Figure 5 . Finally, R8 computes the next hops for the unscoped
destination prefixes. The results are shown in the third table in
Figure 5.
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forwarding entries scoped to
source prefix = 2001:db8:0:a000::/52
============================================
D=2001:db8:0:5555/64 NH=R7
D=::/0 NH=R7
forwarding entries scoped to
source prefix = 2001:db8:0:b000::/52
============================================
D=2001:db8:0:6666/64 NH=SERb2
D=::/0 NH=SERb1
unscoped forwarding entries
============================================
D=2001:db8:0:a010::/64 NH=R2
D=2001:db8:0:b010::/64 NH=R2
D=2001:db8:0:a020::/64 NH=R5
D=2001:db8:0:b020::/64 NH=R5
D=2001:db8:0:5555::/64 NH=R7
D=2001:db8:0:6666::/64 NH=SERb2
D=::/0 NH=SERb1
Figure 5: Forwarding Entries Computed at R8
The final step is for R8 to augment the less specific source-prefix-
scoped forwarding entries with more specific source-prefix-scoped
forwarding entries. As unscoped forwarding table is considered being
scoped to ::/0 and both 2001:db8:0:a000::/52 and 2001:db8:0:b000::/52
are more specific prefixes of ::/0, the unscoped (scoped to ::/0)
forwarding table needs to be augmented with both more specific
source-prefix-scoped tables. If an less specific scoped forwarding
entry has the exact same destination prefix as an more specific
source-prefix-scoped forwarding entry (including destination prefix
length), then the more specific source-prefix-scoped forwarding entry
wins.
As as an example of how the source scoped forwarding entries are
augmented, we consider how the two entries in the first table in
Figure 5 (the table for source prefix = 2001:db8:0:a000::/52) are
augmented with entries from the third table in Figure 5 (the table of
unscoped or scoped for ::/0 forwarding entries). The first four
unscoped forwarding entries (D=2001:db8:0:a010::/64,
D=2001:db8:0:b010::/64, D=2001:db8:0:a020::/64, and
D=2001:db8:0:b020::/64) are not an exact match for any of the
existing entries in the forwarding table for source prefix
2001:db8:0:a000::/52. Therefore, these four entries are added to the
final forwarding table for source prefix 2001:db8:0:a000::/52. The
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result of adding these entries is reflected in first four entries the
first table in Figure 6.
The next less specific scoped (scope is ::/0) forwarding table entry
is for D=2001:db8:0:5555::/64. This entry is an exact match for the
existing entry in the forwarding table for the more specific source
prefix 2001:db8:0:a000::/52. Therefore, we do not replace the
existing entry with the entry from the unscoped forwarding table.
This is reflected in the fifth entry in the first table in Figure 6.
(Note that since both scoped and unscoped entries have R7 as the next
hop, the result of applying this rule is not visible.)
The next less specific prefix scoped (scope is ::/0) forwarding table
entry is for D=2001:db8:0:6666::/64. This entry is not an exact
match for any existing entries in the forwarding table for source
prefix 2001:db8:0:a000::/52. Therefore, we add this entry. This is
reflected in the sixth entry in the first table in Figure 6.
The next less specific prefix scoped (scope is ::/0) forwarding table
entry is for D=::/0. This entry is an exact match for the existing
entry in the forwarding table for more specific source prefix
2001:db8:0:a000::/52. Therefore, we do not overwrite the existing
source-prefix-scoped entry, as can be seen in the last entry in the
first table in Figure 6.
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if source address matches 2001:db8:0:a000::/52
then use this forwarding table
============================================
D=2001:db8:0:a010::/64 NH=R2
D=2001:db8:0:b010::/64 NH=R2
D=2001:db8:0:a020::/64 NH=R5
D=2001:db8:0:b020::/64 NH=R5
D=2001:db8:0:5555::/64 NH=R7
D=2001:db8:0:6666::/64 NH=SERb2
D=::/0 NH=R7
else if source address matches 2001:db8:0:b000::/52
then use this forwarding table
============================================
D=2001:db8:0:a010::/64 NH=R2
D=2001:db8:0:b010::/64 NH=R2
D=2001:db8:0:a020::/64 NH=R5
D=2001:db8:0:b020::/64 NH=R5
D=2001:db8:0:5555::/64 NH=R7
D=2001:db8:0:6666::/64 NH=SERb2
D=::/0 NH=SERb1
else if source address matches ::/0 use this forwarding table
============================================
D=2001:db8:0:a010::/64 NH=R2
D=2001:db8:0:b010::/64 NH=R2
D=2001:db8:0:a020::/64 NH=R5
D=2001:db8:0:b020::/64 NH=R5
D=2001:db8:0:5555::/64 NH=R7
D=2001:db8:0:6666::/64 NH=SERb2
D=::/0 NH=SERb1
Figure 6: Complete Forwarding Tables Computed at R8
The forwarding tables produced by this process at R8 have the desired
properties. A packet with a source address in 2001:db8:0:a000::/52
will be forwarded based on the first table in Figure 6. If the
packet is destined for the Internet at large or the service at
D=2001:db8:0:5555/64, it will be sent to R7 in the direction of SERa.
If the packet is destined for an internal host, then the first four
entries will send it to R2 or R5 as expected. Note that if this
packet has a destination address corresponding to the service offered
by ISP-B (D=2001:db8:0:5555::/64), then it will get forwarded to
SERb2. It will be dropped by SERb2 or by ISP-B, since it the packet
has a source address that was not assigned by ISP-B. However, this
is expected behavior. In order to use the service offered by ISP-B,
the host needs to originate the packet with a source address assigned
by ISP-B.
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In this example, a packet with a source address that doesn't match
2001:db8:0:a000::/52 or 2001:db8:0:b000::/52 must have originated
from an external host. Such a packet will use the unscoped
forwarding table (the last table in Figure 6). These packets will
flow exactly as they would in absence of multihoming.
We can also modify this example to illustrate how it supports
deployments where not all routers in the site support SADR.
Continuing with the topology shown in Figure 3, suppose that R3 and
R5 do not support SADR. Instead they are only capable of
understanding unscoped route advertisements. The SADR routers in the
network will still originate the routes shown in Figure 4. However,
R3 and R5 will only understand the unscoped routes as shown in
Figure 7.
Routes originated by SERa:
(D=2001:db8:0:5555::/64)
(D=::/0)
Routes originated by SERb1:
(D=::/0)
Routes originated by SERb2:
(D=2001:db8:0:6666::/64)
Routes originated by R1:
(D=2001:db8:0:a010::/64)
(D=2001:db8:0:b010::/64)
Routes originated by R2:
(D=2001:db8:0:a010::/64)
(D=2001:db8:0:b010::/64)
Routes originated by R3:
(D=2001:db8:0:a020::/64)
(D=2001:db8:0:b020::/64)
Figure 7: Routes Advertisements Understood by Routers that do no
Support SADR
With these unscoped route advertisements, R5 will produce the
forwarding table shown in Figure 8.
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forwarding table
============================================
D=2001:db8:0:a010::/64 NH=R8
D=2001:db8:0:b010::/64 NH=R8
D=2001:db8:0:a020::/64 NH=R3
D=2001:db8:0:b020::/64 NH=R3
D=2001:db8:0:5555::/64 NH=R8
D=2001:db8:0:6666::/64 NH=SERb2
D=::/0 NH=R8
Figure 8: Forwarding Table For R5, Which Doesn't Understand Source-
Prefix-Scoped Routes
Any traffic that needs to exit the site will eventually hit a SADR-
capable router. Once that traffic enters the SADR-capable domain,
then it will not leave that domain until it exits the site. This
property is required in order to guarantee that there will not be
routing loops involving SADR-capable and non-SADR-capable routers.
Note that the mechanism described here for converting source-prefix-
scoped destination prefix routing advertisements into forwarding
state is somewhat different from that proposed in
[I-D.ietf-rtgwg-dst-src-routing]. The method described in this
document is intended to be easy to understand for network enterprise
operators while at the same time being functionally correct. Another
difference is that the method in this document assumes that source
prefix will not overlap. Other differences between the two
approaches still need to be understood and reconciled.
An interesting side-effect of deploying SADR is if all routers in a
given network support SADR and have a scoped forwarding table, then
the unscoped forwarding table can be eliminated which ensures that
packets with legitimate source addresses only can leave the network
(as there are no scoped forwarding tables for spoofed/bogon source
addresses). It would prevent accidental leaks of ULA/reserved/link-
local sources to the Internet as well as ensures that no spoofing is
possible from the SADR-enabled network.
4. Mechanisms For Hosts To Choose Good Source Addresses In A Multihomed Site
Until this point, we have made the assumption that hosts are able to
choose the correct source address using some unspecified mechanism.
This has allowed us to just focus on what the routers in a multihomed
site network need to do in order to forward packets to the correct
ISP based on source address. Now we look at possible mechanisms for
hosts to choose the correct source address. We also look at what
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role, if any, the routers may play in providing information that
helps hosts to choose source addresses.
Any host that needs to be able to send traffic using the uplinks to a
given ISP is expected to be configured with an address from the
prefix assigned by that ISP. The host will control which ISP is used
for its traffic by selecting one of the addresses configured on the
host as the source address for outgoing traffic. It is the
responsibility of the site network to ensure that a packet with the
source address from an ISP is now sent on an uplink to that ISP.
If all of the ISP uplinks are working, the choice of source address
by the host may be driven by the desire to load share across ISP
uplinks, or it may be driven by the desire to take advantage of
certain properties of a particular uplink or ISP. If any of the ISP
uplinks is not working, then the choice of source address by the host
can determine if packets get dropped.
How a host should make good decisions about source address selection
in a multihomed site is not a solved problem. We do not attempt to
solve this problem in this document. Instead we discuss the current
state of affairs with respect to standardized solutions and
implementation of those solutions. We also look at proposed
solutions for this problem.
An external host initiating communication with a host internal to a
PA multihomed site will need to know multiple addresses for that host
in order to communicate with it using different ISPs to the
multihomed site. These addresses are typically learned through DNS.
(For simplicity, we assume that the external host is single-homed.)
The external host chooses the ISP that will be used at the remote
multihomed site by setting the destination address on the packets it
transmits. For a sessions originated from an external host to an
internal host, the choice of source address used by the internal host
is simple. The internal host has no choice but to use the
destination address in the received packet as the source address of
the transmitted packet.
For a session originated by a host internal to the multi-homed site,
the decision of what source address to select is more complicated.
We consider three main methods for hosts to get information about the
network. The two proactive methods are Neighbor Discovery Router
Advertisements and DHCPv6. The one reactive method we consider is
ICMPv6. Note that we are explicitly excluding the possibility of
having hosts participate in or even listen directly to routing
protocol advertisements.
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First we look at how a host is currently expected to select the
source and destination address with which it sends a packet.
4.1. Source Address Selection Algorithm on Hosts
[RFC6724] defines the algorithms that hosts are expected to use to
select source and destination addresses for packets. It defines an
algorithm for selecting a source address and a separate algorithm for
selecting a destination address. Both of these algorithms depend on
a policy table. [RFC6724] defines a default policy which produces
certain behavior.
The rules in the two algorithms in [RFC6724] depend on many different
properties of addresses. While these are needed for understanding
how a host should choose addresses in an arbitrary environment, most
of the rules are not relevant for understanding how a host should
choose among multiple source addresses in mulitihomed envinronment
when sending a packet to a remote host. Returning to the example in
Figure 3, we look at what the default algorithms in [RFC6724] say
about the source address that internal host H31 should use to send
traffic to external host H101, somewhere on the Internet. Let's look
at what rules in [RFC6724] are actually used by H31 in this case.
There is no choice to be made with respect to destination address.
H31 needs to send a packet with D=2001:db8:0:1234::101 in order to
reach H101. So H31 have to choose between using
S=2001:db8:0:a010::31 or S=2001:db8:0:b010::31 as the source address
for this packet. We go through the rules for source address
selection in Section 5 of [RFC6724]. Rule 1 (Prefer same address) is
not useful to break the tie between source addresses, because neither
the candidate source addresses equals the destination address. Rule
2 (Prefer appropriate scope) is also not used in this scenario,
because both source addresses and the destination address have global
scope.
Rule 3 (Avoid deprecated addresses) applies to an address that has
been autoconfigured by a host using stateless address
autoconfiguration as defined in [RFC4862]. An address autoconfigured
by a host has a preferred lifetime and a valid lifetime. The address
is preferred until the preferred lifetime expires, after which it
becomes deprecated. A deprecated address is not used if there is a
preferred address of the appropriate scope available. When the valid
lifetime expires, the address cannot be used at all. The preferred
and valid lifetimes for an autoconfigured address are set based on
the corresponding lifetimes in the Prefix Information Option in
Neighbor Discovery Router Advertisements. So a possible tool to
control source address selection in this scenario would be for a host
to make an address deprecated by having routers on that link, R1 and
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R2 in Figure 3, send a Router Advertisement message contaning a
Prefix Information Option for the source prefix to be discouraged (or
prohibited) with the preferred lifetime set to zero. This is a
rather blunt tool, because it discourages or prohibits the use of
that source prefix for all destinations. However, it may be useful
in some scenarios. For example, if all uplinks to a particular ISP
fail, it is desirable to prevent hosts from using source addresses
from that ISP address space.
Rule 4 (Avoid home addresses) does not apply here because we are not
considering Mobile IP.
Rule 5 (Prefer outgoing interface) is not useful in this scenario,
because both source addresses are assigned to the same interface.
Rule 5.5 (Prefer addresses in a prefix advertised by the next-hop) is
not useful in the scenario when both R1 and R2 will advertise both
source prefixes. However potentially this rule may allow a host to
select the correct source prefix by selecting a next-hop. The most
obvious way would be to make R1 to advertise itself as a default
router and send PIO for 2001:db8:0:a010::/64, while R2 is advertising
itself as a default router and sending PIO for 2001:db8:0:b010::/64.
We'll discuss later how Rule 5.5 can be used to influence a source
address selection in single-router topologies (e.g. when H41 is
sending traffic using R3 as a default gateway).
Rule 6 (Prefer matching label) refers to the Label value determined
for each source and destination prefix as a result of applying the
policy table to the prefix. With the default policy table defined in
Section 2.1 of [RFC6724], Label(2001:db8:0:a010::31) = 5,
Label(2001:db8:0:b010::31) = 5, and Label(2001:db8:0:1234::101) = 5.
So with the default policy, Rule 6 does not break the tie. However,
the algorithms in [RFC6724] are defined in such as way that non-
default address selection policy tables can be used. [RFC7078]
defines a way to distribute a non-default address selection policy
table to hosts using DHCPv6. So even though the application of rule
6 to this scenario using the default policy table is not useful, rule
6 may still be a useful tool.
Rule 7 (Prefer temporary addresses) has to do with the technique
described in [RFC4941] to periodically randomize the interface
portion of an IPv6 address that has been generated using stateless
address autoconfiguration. In general, if H31 were using this
technique, it would use it for both source addresses, for example
creating temporary addresses 2001:db8:0:a010:2839:9938:ab58:830f and
2001:db8:0:b010:4838:f483:8384:3208, in addition to
2001:db8:0:a010::31 and 2001:db8:0:b010::31. So this rule would
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prefer the two temporary addresses, but it would not break the tie
between the two source prefixes from ISP-A and ISP-B.
Rule 8 (Use longest matching prefix) dictates that between two
candidate source addresses the one which has longest common prefix
length with the destination address. For example, if H31 were
selecting the source address for sending packets to H101, this rule
would not be a tie breaker as for both candidate source addresses
2001:db8:0:a101::31 and 2001:db8:0:b101::31 the common prefix length
with the destination is 48. However if H31 were selecting the source
address for sending packets H41 address 2001:db8:0:a020::41, then
this rule would result in using 2001:db8:0:a101::31 as a source
(2001:db8:0:a101::31 and 2001:db8:0:a020::41 share the common prefix
2001:db8:0:a000::/58, while for `2001:db8:0:b101::31 and
2001:db8:0:a020::41 the common prefix is 2001:db8:0:a000::/51).
Therefore rule 8 might be useful for selecting the correct source
address in some but not all scenarios (for example if ISP-B services
belong to 2001:db8:0:b000::/59 then H31 would always use
2001:db8:0:b010::31 to access those destinations).
So we can see that of the 8 source selection address rules from
[RFC6724], five actually apply to our basic site multihoming
scenario. The rules that are relevant to this scenario are
summarized below.
o Rule 3: Avoid deprecated addresses.
o Rule 5.5: Prefer addresses in a prefix advertised by the next-hop.
o Rule 6: Prefer matching label.
o Rule 8: Prefer longest matching prefix.
The two methods that we discuss for controlling the source address
selection through the four relevant rules above are SLAAC Router
Advertisement messages and DHCPv6.
We also consider a possible role for ICMPv6 for getting traffic-
driven feedback from the network. With the source address selection
algorithm discussed above, the goal is to choose the correct source
address on the first try, before any traffic is sent. However,
another strategy is to choose a source address, send the packet, get
feedback from the network about whether or not the source address is
correct, and try another source address if it is not.
We consider four scenarios where a host needs to select the correct
source address. The first is when both uplinks are working. The
second is when one uplink has failed. The third one is a situation
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when one failed uplink has recovered. The last one is failure of
both (all) uplinks.
4.2. Selecting Source Address When Both Uplinks Are Working
Again we return to the topology in Figure 3. Suppose that the site
administrator wants to implement a policy by which all hosts need to
use ISP-A to reach H01 at D=2001:db8:0:1234::101. So for example,
H31 needs to select S=2001:db8:0:a010::31.
4.2.1. Distributing Address Selection Policy Table with DHCPv6
This policy can be implemented by using DHCPv6 to distribute an
address selection policy table that assigns the same label to
destination address that match 2001:db8:0:1234::/64 as it does to
source addresses that match 2001:db8:0:a000::/52. The following two
entries accomplish this.
Prefix Precedence Label
2001:db8:0:1234::/64 50 33
2001:db8:0:a000::/52 50 33
Figure 9: Policy table entries to implement a routing policy
This requires that the hosts implement [RFC6724], the basic source
and destination address framework, along with [RFC7078], the DHCPv6
extension for distributing a non-default policy table. Note that it
does NOT require that the hosts use DHCPv6 for address assignment.
The hosts could still use stateless address autoconfiguration for
address configuration, while using DHCPv6 only for policy table
distribution (see [RFC3736]). However this method has a number of
disadvantages:
o DHCPv6 support is not a mandatory requirement for IPv6 hosts, so
this method might not work for all devices.
o Network administrators are required to explicitly configure the
desired network access policies on DHCPv6 servers. While it might
be feasible in the scenarion of a single multihomed network, such
approach might have some scalability issues, especially if the
centralized DHCPv6 solution is deployed to serve a large number of
multiomed sites.
4.2.2. Controlling Source Address Selection With Router Advertisements
Neighbor Discovery currently has two mechanisms to communicate prefix
information to hosts. The base specification for Neighbor Discovery
(see [RFC4861]) defines the Prefix Information Option (PIO) in the
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Router Advertisement (RA) message. When a host receives a PIO with
the A-flag set, it can use the prefix in the PIO as source prefix
from which it assigns itself an IP address using stateless address
autoconfiguration (SLAAC) procedures described in [RFC4862]. In the
example of Figure 3, if the site network is using SLAAC, we would
expect both R1 and R2 to send RA messages with PIOs for both source
prefixes 2001:db8:0:a010::/64 and 2001:db8:0:b010::/64 with the
A-flag set. H31 would then use the SLAAC procedure to configure
itself with the 2001:db8:0:a010::31 and 2001:db8:0:b010::31.
Whereas a host learns about source prefixes from PIO messages, hosts
can learn about a destination prefix from a Router Advertisement
containing Route Information Option (RIO), as specified in [RFC4191].
The destination prefixes in RIOs are intended to allow a host to
choose the router that it uses as its first hop to reach a particular
destination prefix.
As currently standardized, neither PIO nor RIO options contained in
Neighbor Discovery Router Advertisements can communicate the
information needed to implement the desired routing policy. PIO's
communicate source prefixes, and RIO communicate destination
prefixes. However, there is currently no standardized way to
directly associate a particular destination prefix with a particular
source prefix.
[I-D.pfister-6man-sadr-ra] proposes a Source Address Dependent Route
Information option for Neighbor Discovery Router Advertisements which
would associate a source prefix and with a destination prefix. The
details of [I-D.pfister-6man-sadr-ra] might need tweaking to address
this use case. However, in order to be able to use Neighbor
Discovery Router Advertisements to implement this routing policy, an
extension that allows a R1 and R2 to explicitly communicate to H31 an
association between S=2001:db8:0:a000::/52 D=2001:db8:0:1234::/64
would be needed.
However, Rule 5.5 of the source address selection algorithm
(discussed in Section 4.1 above), together with default router
preference (specified in [RFC4191]) and RIO can be used to influence
a source address selection on a host as described below. Let's look
at source address selection on the host H41. It receives RAs from R3
with PIOs for 2001:db8:0:a020::/64 and 2001:db8:0:b020::/64. At that
point all traffic would use the same next-hop (R3 link-local address)
so Rule 5.5 does not apply. Now let's assume that R3 supports SADR
and has two scoped forwarding tables, one scoped to
S=2001:db8:0:a000::/52 and another scoped to S=2001:db8:0:b000::/52.
If R3 generates two different link-local addresses for its interface
facing H41 (one for each scoped forwarding table, LLA_A and LLA_B)
and starts sending two different RAs: one is sent from LLA_A and
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includes PIO for 2001:db8:0:a020::/64, another us sent from LLA_B and
includes PIO for 2001:db8:0:b020::/64. Now it is possible to
influence H41 source address selection for destinations which follow
the default route by setting default router preference in RAs. If it
is desired that H41 reaches H101 (or any destinations in the
Internet) via ISP-A, then RAs sent from LLA_A should have default
router preference set to 01 (high priority), while RAs sent from
LLA_B should have preference set to 11 (low). Then LLA_A would be
chosen as a next-hop for H101 and therefore (as per rule 5.5)
2001:db8:0:a020::41 would be selected as the source address. If, at
the same time, it is desired that H61 is accessible via ISP-B then R3
should include a RIO for 2001:db8:0:6666::/64 to its RA sent from
LLA_B. H41 would chose LLA_B as a next-hop for all traffic to H61
and then as per Rule 5.5, 2001:db8:0:b020::41 would be selected as a
source address.
If in the above mentioned scenario it is desirable that all Internet
traffic leaves the network via ISP-A and the link to ISP-B is used
for accessing ISP-B services only (not as ISP-A link backup), then
RAs sent by R3 from LLA_B should have Router Lifetime set to 0 and
should include RIOs for ISP-B address space. It would instruct H41
to use LLA_A for all Internet traffic but use LLA_B as a next-hop
while sending traffic to ISP-B addresses.
The description of the mechanism above assumes SADR support by the
first-hop routers as well as SERs. However, a first-hop router can
still provide a less flexible version of this mechanism even without
implementing SADR. This could be done by providing configuration
knobs on the first-hop router that allow it to generate different
link-local addresses and to send individual RAs for each prefix.
The mechanism described above relies on Rule 5.5 of the default
source address selection algorithm defined in [RFC6724]. [RFC8028]
recommends that a host SHOULD select default routers for each prefix
in which it is assigned an address. It also recommends that hosts
SHOULD implement Rule 5.5. of [RFC6724]. Hosts following the
recommendations specified in [RFC8028] therefore should be able to
benefit from the solution described in this document. No standards
need to be updated in regards to host behavior.
4.2.3. Controlling Source Address Selection With ICMPv6
We now discuss how one might use ICMPv6 to implement the routing
policy to send traffic destined for H101 out the uplink to ISP-A,
even when uplinks to both ISPs are working. If H31 started sending
traffic to H101 with S=2001:db8:0:b010::31 and
D=2001:db8:0:1234::101, it would be routed through SER-b1 and out the
uplink to ISP-B. SERb1 could recognize that this is traffic is not
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following the desired routing policy and react by sending an ICMPv6
message back to H31.
In this example, we could arrange things so that SERb1 drops the
packet with S=2001:db8:0:b010::31 and D=2001:db8:0:1234::101, and
then sends to H31 an ICMPv6 Destination Unreachable message with Code
5 (Source address failed ingress/egress policy). When H31 receives
this packet, it would then be expected to try another source address
to reach the destination. In this example, H31 would then send a
packet with S=2001:db8:0:a010::31 and D=2001:db8:0:1234::101, which
will reach SERa and be forwarded out the uplink to ISP-A.
However, we would also want it to be the case that SERb1 does not
enforce this routing policy when the uplink from SERa to ISP-A has
failed. This could be accomplished by having SERa originate a
source-prefix-scoped route for (S=2001:db8:0:a000::/52,
D=2001:db8:0:1234::/64) and have SERb1 monitor the presence of that
route. If that route is not present (because SERa has stopped
originating it), then SERb1 will not enforce the routing policy, and
it will forward packets with S=2001:db8:0:b010::31 and
D=2001:db8:0:1234::101 out its uplink to ISP-B.
We can also use this source-prefix-scoped route originated by SERa to
communicate the desired routing policy to SERb1. We can define an
EXCLUSIVE flag to be advertised together with the IGP route for
(S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64). This would allow
SERa to communicate to SERb that SERb should reject traffic for
D=2001:db8:0:1234::/64 and respond with an ICMPv6 Destination
Unreachable Code 5 message, as long as the route for
(S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64) is present.
Finally, if we are willing to extend ICMPv6 to support this solution,
then we could create a mechanism for SERb1 to tell the host what
source address it should be using to successfully forward packets
that meet the policy. In its current form, when SERb1 sends an
ICMPv6 Destination Unreachable Code 5 message, it is basically
saying, "This source address is wrong. Try another source address."
In the absence of a clear indication which address to try next, the
host will iterate over all addresses assigned to the interface (e.g.
various privacy addresses) which would lead to significant delays and
degraded user experience. It would be better is if the ICMPv6
message could say, "This source address is wrong. Instead use a
source address in S=2001:db8:0:a000::/52.".
However using ICMPv6 for signalling source address information back
to hosts introduces new challenges. Most routers currently have
software or hardware limits on generating ICMP messages. An site
administrator deploying a solution that relies on the SERs generating
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ICMP messages could try to improve the performance of SERs for
generating ICMP messages. However, in a large network, it is still
likely that ICMP message generation limits will be reached. As a
result hosts would not receive ICMPv6 back which in turn leads to
traffic blackholing and poor user experience. To improve the
scalability of ICMPv6-based signalling hosts SHOULD cache the
preferred source address (or prefix) for the given destination (which
in turn might cause issues in case of the corresponding ISP uplinks
failure - see Section 4.3). In addition, the same source prefix
SHOULD be used for other destinations in the same /64 as the original
destination address. The source prefix SHOULD have a specific
lifetime. Expiration of the lifetime SHOULD trigger the source
address selection algorithm again.
Using ICMPv6 Code 5 message for influencing source address selection
allows an attacker to exhaust the list of candidate source addresses
on the host by sending spoofed ICMPv6 Code 5 for all prefixes known
on the network (therefore preventing a victim from establishing a
communication with the destination host). To protect from such
attack hosts SHOULD verify that the original packet header included
into ICMPv6 error message was actually sent by the host.
As currently standardized in [RFC4443], the ICMPv6 Destination
Unreachable Message with Code 5 would allow for the iterative
approach to retransmitting packets using different source addresses.
As currently defined, the ICMPv6 message does not provide a mechanism
to communication information about which source prefix should be used
for a retransmitted packet. The current document does not define
such a mechanism. However, we note that this might be a useful
extension to define in a different document.
4.2.4. Summary of Methods For Controlling Source Address Selection To Implement Routing Policy
So to summarize this section, we have looked at three methods for
implementing a simple routing policy where all traffic for a given
destination on the Internet needs to use a particular ISP, even when
the uplinks to both ISPs are working.
The default source address selection policy cannot distinguish
between the source addresses needed to enforce this policy, so a non-
default policy table using associating source and destination
prefixes using Label values would need to be installed on each host.
A mechanism exists for DHCPv6 to distribute a non-default policy
table but such solution would heavily rely on DHCPv6 support by host
operating system. Moreover there is no mechanism to translate
desired routing/traffic engineering policies into policy tables on
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DHCPv6 servers. Therefore using DHCPv6 for controlling address
selection policy table is not recommended and SHOULD NOT be used.
At the same time Router Advertisements provide a reliable mechanism
to influence source address selection process via PIO, RIO and
default router preferences. As all those options have been
standardized by IETF and are supported by various operating systems,
no changes are required on hosts. First-hop routers in the
enterprise network need to be able of sending different RAs for
different SLAAC prefixes (either based on scoped forwarding tables or
based on pre-configured policies).
SERs can enforce the routing policy by sending ICMPv6 Destination
Unreachable messages with Code 5 (Source address failed ingress/
egress policy) for traffic that is being sent with the wrong source
address. The policy distribution can be automated by defining an
EXCLUSIVE flag for the source-prefix-scoped route which can be set on
the SER that originates the route. As ICMPv6 message generation can
be rate-limited on routers, it SHOULD NOT be used as the only
mechanism to influence source address selection on hosts. While
hosts SHOULD select the correct source address for a given
destination the network SHOULD signal any source address issues back
to hosts using ICMPv6 error messages.
4.3. Selecting Source Address When One Uplink Has Failed
Now we discuss if DHCPv6, Neighbor Discovery Router Advertisements,
and ICMPv6 can help a host choose the right source address when an
uplink to one of the ISPs has failed. Again we look at the scenario
in Figure 3. This time we look at traffic from H31 destined for
external host H501 at D=2001:db8:0:5678::501. We initially assume
that the uplink from SERa to ISP-A is working and that the uplink
from SERb1 to ISP-B is working.
We assume there is no particular routing policy desired, so H31 is
free to send packets with S=2001:db8:0:a010::31 or
S=2001:db8:0:b010::31 and have them delivered to H501. For this
example, we assume that H31 has chosen S=2001:db8:0:b010::31 so that
the packets exit via SERb to ISP-B. Now we see what happens when the
link from SERb1 to ISP-B fails. How should H31 learn that it needs
to start sending the packet to H501 with S=2001:db8:0:a010::31 in
order to start using the uplink to ISP-A? We need to do this in a
way that doesn't prevent H31 from still sending packets with
S=2001:db8:0:b010::31 in order to reach H61 at D=2001:db8:0:6666::61.
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For this example we assume that the site network in Figure 3 has a
centralized DHCP server and all routers act as DHCP relay agents. We
assume that both of the addresses assigned to H31 were assigned via
DHCP.
We could try to have the DHCP server monitor the state of the uplink
from SERb1 to ISP-B in some manner and then tell H31 that it can no
longer use S=2001:db8:0:b010::31 by settings its valid lifetime to
zero. The DHCP server could initiate this process by sending a
Reconfigure Message to H31 as described in Section 19 of [RFC3315].
Or the DHCP server can assign addresses with short lifetimes in order
to force clients to renew them often.
This approach would prevent H31 from using S=2001:db8:0:b010::31 to
reach the a host on the Internet. However, it would also prevent H31
from using S=2001:db8:0:b010::31 to reach H61 at
D=2001:db8:0:6666::61, which is not desirable.
Another potential approach is to have the DHCP server monitor the
uplink from SERb1 to ISP-B and control the choice of source address
on H31 by updating its address selection policy table via the
mechanism in [RFC7078]. The DHCP server could initiate this process
by sending a Reconfigure Message to H31. Note that [RFC3315]
requires that Reconfigure Message use DHCP authentication. DHCP
authentication could be avoided by using short address lifetimes to
force clients to send Renew messages to the server often. If the
host is not obtaining its IP addresses from the DHCP server, then it
would need to use the Information Refresh Time option defined in
[RFC4242].
If the following policy table can be installed on H31 after the
failure of the uplink from SERb1, then the desired routing behavior
should be achieved based on source and destination prefix being
matched with label values.
Prefix Precedence Label
::/0 50 44
2001:db8:0:a000::/52 50 44
2001:db8:0:6666::/64 50 55
2001:db8:0:b000::/52 50 55
Figure 10: Policy Table Needed On Failure Of Uplink From SERb1
The described solution has a number of significant drawbacks, some of
them already discussed in Section 4.2.1.
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o DHCPv6 support is not required for an IPv6 host and there are
operating systems which do not support DHCPv6. Besides that, it
does not appear that [RFC7078] has been widely implemented on host
operating systems.
o [RFC7078] does not clearly specify this kind of a dynamic use case
where address selection policy needs to be updated quickly in
response to the failure of a link. In a large network it would
present scalability issues as many hosts need to be reconfigured
in very short period of time.
o Updating DHCPv6 server configuration each time an ISP uplink
changes its state introduces some scalability issues, especially
for mid/large distributed scale enterprise networks. In addition
to that, the policy table needs to be manually configured by
administrators which makes that solution prone to human error.
o No mechanism exists for making DHCPv6 servers aware of network
topology/routing changes in the network. In general DHCPv6
servers monitoring network-related events sounds like a bad idea
as completely new functionality beyond the scope of DHCPv6 role is
required.
4.3.2. Controlling Source Address Selection With Router Advertisements
The same mechanism as discussed in Section 4.2.2 can be used to
control the source address selection in the case of an uplink
failure. If a particular prefix should not be used as a source for
any destinations, then the router needs to send RA with Preferred
Lifetime field for that prefix set to 0.
Let's consider a scenario when all uplinks are operational and H41
receives two different RAs from R3: one from LLA_A with PIO for
2001:db8:0:a020::/64, default router preference set to 11 (low) and
another one from LLA_B with PIO for 2001:db8:0:a020::/64, default
router preference set to 01 (high) and RIO for 2001:db8:0:6666::/64.
As a result H41 is using 2001:db8:0:b020::41 as a source address for
all Internet traffic and those packets are sent by SERs to ISP-B. If
SERb1 uplink to ISP-B failed, the desired behavior is that H41 stops
using 2001:db8:0:b020::41 as a source address for all destinations
but H61. To achieve that R3 should react to SERb1 uplink failure
(which could be detected as the scoped route (S=2001:db8:0:b000::/52,
D=::/0) disappearance) by withdrawing itself as a default router. R3
sends a new RA from LLA_B with Router Lifetime value set to 0 (which
means that it should not be used as default router). That RA still
contains PIO for 2001:db8:0:b020::/64 (for SLAAC purposes) and RIO
for 2001:db8:0:6666::/64 so H41 can reach H61 using LLA_B as a next-
hop and 2001:db8:0:b020::41 as a source address. For all traffic
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following the default route, LLA_A will be used as a next-hop and
2001:db8:0:a020::41 as a source address.
If all uplinks to ISP-B have failed and therefore source addresses
from ISP-B address space should not be used at all, the forwarding
table scoped S=2001:db8:0:b000::/52 contains no entries. Hosts can
be instructed to stop using source addresses from that block by
sending RAs containing PIO with Preferred Lifetime set to 0.
4.3.3. Controlling Source Address Selection With ICMPv6
Now we look at how ICMPv6 messages can provide information back to
H31. We assume again that at the time of the failure H31 is sending
packets to H501 using (S=2001:db8:0:b010::31,
D=2001:db8:0:5678::501). When the uplink from SERb1 to ISP-B fails,
SERb1 would stop originating its source-prefix-scoped route for the
default destination (S=2001:db8:0:b000::/52, D=::/0) as well as its
unscoped default destination route. With these routes no longer in
the IGP, traffic with (S=2001:db8:0:b010::31, D=2001:db8:0:5678::501)
would end up at SERa based on the unscoped default destination route
being originated by SERa. Since that traffic has the wrong source
address to be forwarded to ISP-A, SERa would drop it and send a
Destination Unreachable message with Code 5 (Source address failed
ingress/egress policy) back to H31. H31 would then know to use
another source address for that destination and would try with
(S=2001:db8:0:a010::31, D=2001:db8:0:5678::501). This would be
forwarded to SERa based on the source-prefix-scoped default
destination route still being originated by SERa, and SERa would
forward it to ISP-A. As discussed above, if we are willing to extend
ICMPv6, SERa can even tell H31 what source address it should use to
reach that destination. The expected host behaviour has been
discussed in Section 4.2.3. Potential issue with using ICMPv6 for
signalling source address issues back to hosts is that uplink to an
ISP-B failure immediately invalidates source addresses from
2001:db8:0:b000::/52 for all hosts which triggers a large number of
ICMPv6 being sent back to hosts - the same scalability/rate limiting
issues discussed in Section 4.2.3 would apply.
4.3.4. Summary Of Methods For Controlling Source Address Selection On The Failure Of An Uplink
It appears that DHCPv6 is not particularly well suited to quickly
changing the source address used by a host in the event of the
failure of an uplink, which eliminates DHCPv6 from the list of
potential solutions. On the other hand Router Advertisements
provides a reliable mechanism to dynamically provide hosts with a
list of valid prefixes to use as source addresses as well as prevent
particular prefixes to be used. While no additional new features are
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required to be implemented on hosts, routers need to be able to send
RAs based on the state of scoped forwarding tables entries and to
react to network topology changes by sending RAs with particular
parameters set.
The use of ICMPv6 Destination Unreachable messages generated by the
SER (or any SADR-capable) routers seem like they have the potential
to provide a support mechanism together with RAs to signal source
address selection errors back to hosts, however scalability issues
may arise in large networks in case of sudden topology change.
Therefore it is highly desirable that hosts are able to select the
correct source address in case of uplinks failure with ICMPv6 being
an additional mechanism to signal unexpected failures back to hosts.
The current behavior of different host operating system when
receiving ICMPv6 Destination Unreachable message with code 5 (Source
address failed ingress/egress policy) is not clear to the authors.
Information from implementers, users, and testing would be quite
helpful in evaluating this approach.
4.4. Selecting Source Address Upon Failed Uplink Recovery
The next logical step is to look at the scenario when a failed uplink
on SERb1 to ISP-B is coming back up, so hosts can start using source
addresses belonging to 2001:db8:0:b000::/52 again.
4.4.1. Controlling Source Address Selection With DHCPv6
The mechanism to use DHCPv6 to instruct the hosts (H31 in our
example) to start using prefixes from ISP-B space (e.g.
S=2001:db8:0:b010::31 for H31) to reach hosts on the Internet is
quite similar to one discussed in Section 4.3.1 and shares the same
drawbacks.
4.4.2. Controlling Source Address Selection With Router Advertisements
Let's look at the scenario discussed in Section 4.3.2. If the
uplink(s) failure caused the complete withdrawal of prefixes from
2001:db8:0:b000::/52 address space by setting Preferred Lifetime
value to 0, then the recovery of the link should just trigger new RA
being sent with non-zero Preferred Lifetime. In another scenario
discussed in Section 4.3.2, the SERb1 uplink to ISP-B failure leads
to disappearance of the (S=2001:db8:0:b000::/52, D=::/0) entry from
the forwarding table scoped to S=2001:db8:0:b000::/52 and, in turn,
caused R3 to send RAs from LLA_B with Router Lifetime set to 0. The
recovery of the SERb1 uplink to ISP-B leads to
(S=2001:db8:0:b000::/52, D=::/0) scoped forwarding entry re-
appearance and instructs R3 that it should advertise itself as a
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default router for ISP-B address space domain (send RAs from LLA_B
with non-zero Router Lifetime).
4.4.3. Controlling Source Address Selection With ICMP
It looks like ICMPv6 provides a rather limited functionality to
signal back to hosts that particular source addresses have become
valid again. Unless the changes in the uplink state a particular
(S,D) pair, hosts can keep using the same source address even after
an ISP uplink has come back up. For example, after the uplink from
SERb1 to ISP-B had failed, H31 received ICMPv6 Code 5 message (as
described in Section 4.3.3) and allegedly started using
(S=2001:db8:0:a010::31, D=2001:db8:0:5678::501) to reach H501. Now
when the SERb1 uplink comes back up, the packets with that (S,D) pair
are still routed to SERa1 and sent to the Internet. Therefore H31 is
not informed that it should stop using 2001:db8:0:a010::31 and start
using 2001:db8:0:b010::31 again. Unless SERa has a policy configured
to drop packets (S=2001:db8:0:a010::31, D=2001:db8:0:5678::501) and
send ICMPv6 back if SERb1 uplink to ISP-B is up, H31 will be unaware
of the network topology change and keep using S=2001:db8:0:a010::31
for Internet destinations, including H51.
One of the possible option may be using a scoped route with EXCLUSIVE
flag as described in Section 4.2.3. SERa1 uplink recovery would
cause (S=2001:db8:0:a000::/52, D=2001:db8:0:1234::/64) route to
reappear in the routing table. In the absence of that route packets
to H101 which were sent to ISP-B (as ISP-A uplink was down) with
source addresses from 2001:db8:0:b000::/52. When the route re-
appears SERb1 would reject those packets and sends ICMPv6 back as
discussed in Section 4.2.3. Practically it might lead to scalability
issues which have been already discussed in Section 4.2.3 and
Section 4.4.3.
4.4.4. Summary Of Methods For Controlling Source Address Selection Upon Failed Uplink Recovery
Once again DHCPv6 does not look like reasonable choice to manipulate
source address selection process on a host in the case of network
topology changes. Using Router Advertisement provides the flexible
mechanism to dynamically react to network topology changes (if
routers are able to use routing changes as a trigger for sending out
RAs with specific parameters). ICMPv6 could be considered as a
supporting mechanism to signal incorrect source address back to hosts
but should not be considered as the only mechanism to control the
address selection in multihomed environments.
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One particular tricky case is a scenario when all uplinks have
failed. In that case there is no valid source address to be used for
any external destinations while it might be desirable to have intra-
site connectivity.
4.5.1. Controlling Source Address Selection With DHCPv6
From DHCPv6 perspective uplinks failure should be treated as two
independent failures and processed as described in Section 4.3.1. At
this stage it is quite obvious that it would result in quite
complicated policy table which needs to be explicitly configured by
administrators and therefore seems to be impractical.
4.5.2. Controlling Source Address Selection With Router Advertisements
As discussed in Section 4.3.2 an uplink failure causes the scoped
default entry to disappear from the scoped forwarding table and
triggers RAs with zero Router Lifetime. Complete disappearance of
all scoped entries for a given source prefix would cause the prefix
being withdrawn from hosts by setting Preferred Lifetime value to
zero in PIO. If all uplinks (SERa, SERb1 and SERb2) failed, hosts
either lost their default routers and/or have no global IPv6
addresses to use as a source. (Note that 'uplink failure' might mean
'IPv6 connectivity failure with IPv4 still being reachable', in which
case hosts might fall back to IPv4 if there is IPv4 connectivity to
destinations). As a results intra-site connectivity is broken. One
of the possible way to solve it is to use ULAs.
All hosts have ULA addresses assigned in addition to GUAs and used
for intra-site communication even if there is no GUA assigned to a
host. To avoid accidental leaking of packets with ULA sources SADR-
capable routers SHOULD have a scoped forwarding table for ULA source
for internal routes but MUST NOT have an entry for D=::/0 in that
table. In the absence of (S=ULA_Prefix; D=::/0) first-hop routers
will send dedicated RAs from a unique link-local source LLA_ULA with
PIO from ULA address space, RIO for the ULA prefix and Router
Lifetime set to zero. The behaviour is consistent with the situation
when SERb1 lost the uplink to ISP-B (so there is no Internet
connectivity from 2001:db8:0:b000::/52 sources) but those sources can
be used to reach some specific destinations. In the case of ULA
there is no Internet connectivity from ULA sources but they can be
used to reach another ULA destinations. Note that ULA usage could be
particularly useful if all ISPs assign prefixes via DHCP-PD. In the
absence of ULAs uplinks failure hosts would lost all their GUAs upon
prefix lifetime expiration which again makes intra-site communication
impossible.
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It should be noted that the Rule 5.5 (prefer a prefix advertised by
the selected next-hop) takes precedence over the Rule 6 (prefer
matching label, which ensures that GUA source addresses are preferred
over ULAs for GUA destinations). Therefore if ULAs are used, the
network adminstrator needs to ensure that while the site has an
Internet connectivity, hosts do not select a roter which advertises
ULA prefixes as their default router.
4.5.3. Controlling Source Address Selection With ICMPv6
In case of all uplinks failure all SERs will drop outgoing IPv6
traffic and respond with ICMPv6 error message. In the large network
when many hosts are trying to reach Internet destinations it means
that SERs need to generate an ICMPv6 error to every packet they
receive from hosts which presents the same scalability issues
discussed in Section 4.3.34.5.4. Summary Of Methods For Controlling Source Address Selection When All Uplinks Failed
Again, combining SADR with Router Advertisements seems to be the most
flexible and scalable way to control the source address selection on
hosts.
4.6. Summary Of Methods For Controlling Source Address Selection
To summarize the scenarios and options discussed above:
While DHCPv6 allows administrators to manipulate source address
selection policy tables, this method has a number of significant
disadvantages which eliminates DHCPv6 from a list of potential
solutions:
1. It required hosts to support DHCPv6 and its extension (RFC7078);
2. DHCPv6 server needs to monitor network state and detect routing
changes.
3. The use of policy tables requires manual configuration and might
be extremely complicated, especially in the case of distributed
network when large number of remote sites are being served by
centralized DHCPv6 servers.
4. Network topology/routing policy changes could trigger
simultaneous re-configuration of large number of hosts which
present serious scalability issues.
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The use of Router Advertisements to influence the source address
selection on hosts seem to be the most reliable, flexible and
scalable solution. It has the following benefits:
1. no new (non-standard) functionality needs to be implemented on
hosts (except for [RFC4191] support);
2. no changes in RA format;
3. routers can react to routing table changes by sending RAs which
would minimize the failover time in the case of network topology
changes;
4. information required for source address selection is broadcast to
all affected hosts in case of topology change event which
improves the scalability of the solution (comparing to DHCPv6
reconfiguration or ICMPv6 error messages).
To fully benefit from the RA-based solution, first-hop routers need
to implement SADR and be able to send dedicated RAs per scoped
forwarding table as discussed above, reacting to network changes with
sending new RAs. It should be noted that the proposed solution would
work even if first-hop routers are not SADR-capable but still able to
send individual RAs for each ISP prefix and react to topology changes
as discussed above (e.g. via configuration knobs).
The RA-based solution relies heavily on hosts correctly implementing
default address selection algorith as defined in [RFC6724]. While
the basic (and most common) multihoming scenario (two or more
Internet uplinks, no 'wall gardens') would work for any host
supporting the minimal implementation of [RFC6724], more complex use
cases (such as "wall garden" and other scenarios when some ISP
resources can only be reached from that ISP address space) require
that hosts support Rule 5.5 of the default address selection
algorithm. There is some evidence that not all host OSes have that
rule implemented currently. However it should be noted that
[RFC8028] states that Rule 5.5 SHOULD be implemented.
ICMPv6 Code 5 error message SHOULD be used to complement RA-based
solution to signal incorrect source address selection back to hosts,
but it SHOULD NOT be considered as the stand-alone solution. To
prevent scenarios when hosts in multihomed envinronments incorrectly
identify onlink/offlink destinations, hosts should treat ICMPv6
Redirects as discussed in [RFC8028].
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Internet-Draft Enterprise PA Multihoming May 20184.7. Other Configuration Parameters4.7.1. DNS Configuration
In mutihomed envinronment each ISP might provide their own list of
DNS servers. E.g. in the topology show on Figure 3, ISP-A might
provide recursive DNS server H51 2001:db8:0:5555::51, while ISP-B
might provide H61 2001:db8:0:6666::61 as a recursive DNS server.
[RFC8106] defines IPv6 Router Advertisement options to allow IPv6
routers to advertise a list of DNS recursive server addresses and a
DNS Search List to IPv6 hosts. Using RDNSS together with 'scoped'
RAs as described above would allow a first-hop router (R3 in the
Figure 3) to send DNS server addresses and search lists provided by
each ISP (or the corporate DNS servers addresses if the enterprise is
running its own DNS servers).
As discussed in Section 4.5.2, failure of all ISP uplinks would cause
deprecaction of all addresses assigned to a host from the address
space if all ISPs. If any intra-site IPv6 connectivity is still
desirable (most likely to be the case for any mid/large scare
network), then ULAs should be used as discussed in Section 4.5.2. In
such a scenario, the enterprise network should run its own recursive
DNS server(s) and provide its ULA addresses to hosts via RDNSS in RAs
send for ULA-scoped forwarding table as described in Section 4.5.2.
There are some scenarios when the final outcome of the name
resolution might be different depending on:
o which DNS server is used;
o which source address the client uses to send a DNS query to the
server (DNS split horizon).
There is no way currently to instruct a host to use a particular DNS
server out of the configured servers list for resolving a particular
name. Therefore it does not seem feasible to solve the problem of
DNS server selection on the host (it should be noted that this
particular issue is protocol-agnostic and happens for IPv4 as well).
In such a scenario it is recommended that the enterprise run its own
local recursive DNS server.
To influence host source address selection for packets sent to a
particular DNS server the following requirements must be met:
o the host supports RIO as defined in [RFC4191];
o the routers send RIO for routes to DNS server addresses.
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For example, if it is desirable that host H31 reaches the ISP-A DNS
server H51 2001:db8:0:5555::51 using its source address
2001:db8:0:a010::31, then both R1 and R2 should send the RIO
containing the route to 2001:db8:0:5555::51 (or covering route) in
their 'scoped' RAs, containing LLA_A as the default router address
and the PO for SLAAC prefix 2001:db8:0:a010::/64. In that case the
host H31 (if it supports the Rule 5.5) would select LLA_A as a next-
hop and then chose 2001:db8:0:a010::31 as the source address for
packets to the DNS server.
It should be noted that [RFC8106] explicitly prohibits using DNS
information if the RA router Lifetime expired: "An RDNSS address or a
DNSSL domain name MUST be used only as long as both the RA router
Lifetime (advertised by a Router Advertisement message) and the
corresponding option Lifetime have not expired.". Therefore hosts
might ignore RDNSS information provided in ULA-scoped RAs as those
RAs would have router lifetime set to 0. However the updated version
of RFC6106 ([RFC8106]) has that requirement removed.
5. Deployment Considerations
The solution described in this document requires certain mechanisms
to be supported by the network infrastructure and hosts. It requires
some routers in the enterprise site to support some form of Source
Address Dependent Routing (SADR). It also requires hosts to be able
to learn when the uplink to an ISP chages its state so the
corresponding source addresses should (or should not) be used.
Ongoing work to create mechanisms to accomplish this are discussed in
this document, but they are still a work in progress.
The solution discussed in this document relies on the default address
selection algorithm ([RFC6724]) Rule 5.5. While [RFC6724] consideres
this rule as optional, the recent [RFC8028] recommends that a host
SHOULD select default routers for each prefix in which it is assigned
an address. It also recommends that hosts SHOULD implement Rule 5.5.
of [RFC6724]. Therefore while RFC8028-compliant hosts already have
mechanism to learn about ISP uplinks state changes and selecting the
source addresses accordingly, many hosts do not have such mechanism
supported yet.
It should be noted that multihomed enterprise network utilizing
multipe ISP prefixes can be considered as a typical mutiple
provisioning domain (mPVD) scenario, as desribed in [RFC7556]. This
document defines a way for network to provide the PVD information to
hosts indirectly, using the existing mechanisms. At the same time
[I-D.ietf-intarea-provisioning-domains]takes one step further and
describes a comprehensive mechanism for hosts to discover the whole
set of configuration information associated with different PVD/ISPs.
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[I-D.ietf-intarea-provisioning-domains] complements this document in
terms of making hosts being able to learn about ISP uplink states and
selecting the corresponding source addresses.
6. Other Solutions6.1. Shim6
The Shim6 working group specified the Shim6 protocol [RFC5533] which
allows a host at a multihomed site to communicate with an external
host and exchange information about possible source and destination
address pairs that they can use to communicate. It also specified
the REAP protocol [RFC5534] to detect failures in the path between
working address pairs and find new working address pairs. A
fundamental requirement for Shim6 is that both internal and external
hosts need to support Shim6. That is, both the host internal to the
multihomed site and the host external to the multihomed site need to
support Shim6 in order for there to be any benefit for the internal
host to run Shim6. The Shim6 protocol specification was published in
2009, but it has not been widely implemented. Therefore Shim6 is not
considered as a viable solution for enterprise multihoming.
6.2. IPv6-to-IPv6 Network Prefix Translation
IPv6-to-IPv6 Network Prefix Translation (NPTv6) [RFC6296] is not the
focus of this document. NPTv6 suffers from the same fundamental
issue as any other address translation approaches: it breaks end-to-
end connectivity. Therefore NPTv6 is not considered as desirable
solution and this document intentionally focuses on solving
enterprise multihoming problem without any form of address
translations.
With increasing interest and ongoing work in bringing path awareness
to transport and application layer protocols hosts migth be able to
determine the properties of the various network paths and choose
among paths available to them. As selecting the correct source
address is one of the possible mechanisms path-aware hosts may
utilize, address translation negatively affects hosts path-awareness
which makes NTPv6 even more undesirable solution.
6.3. Multipath Transport
Using multipath transport might solve the problems discussed in
Section 4 it would allow hosts to use multiple source addresses for a
single connection and switch between source addresses when a
particular address becomes unavailable or a new address gets assigned
to the host interface. Therefore if all hosts in the enterprise
network are only using multipath transport for all connections, the
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signalling solution described in Section 4 migth not be needed (it
should be noted that the Source Address Dependent Routing would still
be required to delver packets to the correct uplinks). Unfortunatelt
when this document was written, mutlipath transport alone can not be
considered a solition for the problem of selecting the source address
in a multihomed envinronments. There are significant number of hosts
which do not use mulipath transport currently and it seems unlikely
that the situation is going to change in any foreseeable future. As
the solution for enterprise multihoming needs to work for the least
common denominator: hosts without multipath transport support. In
addition, not all protocols are using multipath transport. While
multipath transport would complement the solution described in
Section 4, it could not be considered as a sole solution to the
problem of source address selection in multihomed envinronments.
7. IANA Considerations
This memo asks the IANA for no new parameters.
8. Security Considerations
This document introduces no new security or privacy considerations.
Security considerations of using stateless address autoconfiguration
is discussed in [RFC4862].
9. Acknowledgements
The original outline was suggested by Ole Troan.
The authors would like to thank the following people (in alphabetical
order) for their review and feedback: Olivier Bonaventure, Brian E
Carpenter, Lorenzo Colitti, David Lamparter, Acee Lindem, Philip
Matthewsu, Robert Raszuk, Dave Thaler.
10. References10.1. Normative References
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<https://www.rfc-editor.org/info/rfc1123>.
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